There is a need for portable, low-cost lidar systems that can be used for cloud, aerosols and chemical monitoring from a stand-off distance. At the University of Hawaii we have developed lidar systems based on a 12.7-cm diameter telescope and a 20 Hz frequency-doubled Nd:YAG laser source. For stand off Raman detection of organic liquid and vapors, and plastic explosives, we are using a 0.25-m HoloSpec f/2.2 spectrometer equipped with a gated intensified detector (PI Model I-MAX-1024-E). The samples of interest are excited with 532-nm laser light (35 mJ/pulse). The operational range of the Raman system is in 10's of meters and has been tested at distance of 66 m. This system can also be operated as a Raman lidar by using appropriate filters for atmospheric nitrogen, oxygen and other gaseous species of interest. The Mie-Rayleigh lidar system uses the same telescope and laser, but we have three (1064, 532 and 355-nm) wavelengths available for monitoring clouds and aerosols. A small Hamamatsu H6779 photomultiplier tube (PMT) located near the focal point of telescope detects 532-nm backscatter signal. An avalanche photodiode (APD, EG & G C3095) detector equipped with a 2.5-cm diameter aspheric lens is used for detecting 1064-nm backscatter. The Mie-Rayleigh lidar has usable range of 60 - 4000 m. Results obtained with this system for marine aerosols and clouds are discussed.

A CCD based bistatic lidar (CLidar) system has been developed and constructed to measure scattering in the atmospheric boundary layer. The system used is based on a CCD camera, wide-angle optics and laser. Measuring near the ground with the standard monostatic lidar method is difficult due to the huge change in signal strength with altitude and the incomplete overlap between the laser and the telescope. High spatial (altitude) resolution is also desired near the ground for comparison with in-situ aerosol instruments. Imaging a vertical laser beam from the side with a CCD camera and wide-angle field of view optics overcomes both of these problems. While the molecular signal changes many orders of magnitude in the standard method, it only changes about one order with the CLidar method. In addition, the CLidar resolution near the ground is less than a meter. For perpendicular polarization, the molecular signal is nearly constant all the way to the ground. Other advantages of the CLidar method include low cost and simplicity. The signal is integrated on the CCD rather than with specialized electronics. With the bistatic CLidar method the scattering angle changes with altitude. The variation of scattering intensity with the scattering angle will be influenced by the aerosol size distribution and thus could help provide information on aerosol parameters of interest in the boundary layer.

An inexpensive aerosol lidar is proposped by combining inexpensive fiber-coupled diode laser and photon counting detection technology for air quality monitoring in metropolitan area. A prototype lidar system was built with a 100 μm fiber-coupled diode laser as the laser source and a 200 mm Schmidt-Cassegrain telescope as the receiver. The transmitted beam diveregence from a 50 mm diameter Galilean transmitter was 433 μradian and the receiver field of view was 133 μradian in full angle. With a repetition rate of 5 kHz of 100 ns pulse, a signal-to-noise of 2 was obtained up to 2 km in a clear day nighttime at the averaging time of 60 s.

A satellite-based UV-DIAL measurement system would allow continuous global monitoring of ozone concentration in the upper atmosphere. However such systems remain difficult to implement because aerosol-scattering return signals for satellite-based lidars are very weak. A suitable system must produce high-energy UV pulses at multiple wavelengths with very high efficiency. For example, a nanosecond system operating at 10 Hz must generate approximately 1 J per pulse at 308-320 nm. An efficient space-qualified wavelength-agile system based on a single UV source that can meet this requirement is probably not available using current laser technology. As an alternative, we're pursuing a multi-source approach employing all-solid-state modules that individually generate 300-320 nm light with pulse energies in the range of 50-200 mJ, with transform-limited bandwidths and good beam quality. Pulses from the individual sources can be incoherently summed to obtain the required single-pulse energy. These sources use sum-frequency mixing of the 532 nm second harmonic of an Nd:YAG pump laser with 731-803 nm light derived from a recently-developed, state-of-the-art, nanosecond optical parametric oscillator. Two source configurations are under development, one using extra-cavity sum-frequency mixing, and the other intra-cavity sum-frequency mixing. In either configuration, we hope to obtain sum-frequency mixing efficiency approaching 60% by carefully matching the spatial and temporal properties of the laser and OPO pulses. This ideal balance of green and near-IR photons requires an injection-seeded Nd:YAG pump-laser with very high beam quality, and an OPO exhibiting unusually high conversion efficiency and exceptional signal beam quality. The OPO employs a singly-resonant high-Fresnel-number image-rotating self-injection-seeded nonplanar-ring cavity that achieves pump depletion > 65% and produces signal beams with M² ≈ 3 at pulse energies exceeding 50 mJ. Pump beam requirements can be met in the laboratory using a commercial Nd:YAG laser system, but only after extensive modifications.

A tunable continuous-wave (CW) intracavity pumped periodically poled lithium niobate (PPLN) optical parametric oscillator (OPO) has been developed where a diode-pumped ring-cavity Nd:YAG laser is used as the pumping source. The idler tunable range from 2.3 μm to 3.9 μm with linewidth less than 15 MHz has been demonstrated. The slop efficiency of the idler output versus the diode pump power is ~ 5.6%. The idler output power at 3.4 μm reaches 370 mW when the diode output power is 21.5 W. The PPLN OPO will be applied to seed ZnGeP₂ OPO pumped by a Tm:Ho:YLF laser (λ=2.05 μm). The ZnGeP₂ OPO can be tuned between 3-10.5 μm. Combined PPLN OPO and ZnGeP₂ OPO, the tunable range covers the strong absorption lines of most atmospheric pollutants, and overlaps the mid-infrared atmospheric windows of 3.4-5 μm and 8-13 μm. The mid-infrared emission source is a potential lidar transmitter for remote sensing applications.

Measurements of water vapor profiles over the Southern Great Plains acquired by two different lidars are presented. NASA's airborne DIAL Lidar Atmospheric Sensing Experiment (LASE) system measured water vapor, aerosol, and cloud profiles during the ARM/FIRE Water Vapor Experiment (AFWEX) in November-December 2000 and during the International H2O Project (IHOP) in May-June 2002. LASE measurements acquired during AFWEX are used to characterize upper troposphere water vapor measured by ground-based Raman lidars, radiosondes, and in situ aircraft sensors. LASE measurements acquired during IHOP are being used to better understand the influence water vapor variability on the initiation of deep convection and to improve the quantification and prediction of precipitation associated with these storms. The automated Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) Cloud and Radiation Testbed (CART) Raman Lidar (CARL) has been routinely measuring profiles of water vapor mixing ratio, relative humidity, aerosol extinction, aerosol backscattering, and aerosol and cloud depolarization during both daytime and nighttime operations. Aerosol and water vapor profiles acquired since March 1998 are used to investigate the seasonal variability of the vertical distributions of water vapor and aerosols.

Differential absorption lidar (DIAL) and rotational Raman lidar allow measurements of tropospheric water vapor and temperature, respectively, with very high resolution. So far, these techniques have not yet been combined. In this contribution we present the concept for a lidar system, which employs these two techniques simultaneously and discuss synergetic effects. The combination allows relative humidity profiling with unprecedented resolution and accuracy. In addition, temperature, particle extinction coefficient, and particle backscatter coefficient data measured with rotational Raman lidar can be used to correct the water vapor DIAL data. These corrections are required when gradients in the backscatter ratio exist. We propose to transmit three laser wavelengths and to employ 6 receiver channels. Two infrared wavelengths are transmitted for the water vapor DIAL measurements and one ultraviolet wavelength for the rotational Raman measurements. 3 receiver channels detect the elastic backscatter signals, two channels are used for the detection of the ultraviolet rotational Raman backscatter signals with different temperature-dependency, and one channel is for a temperature-independent rotational Raman backscatter signal which is excited by the off-line infrared laser wavelength.

Knowledge of the spatial and temporal distribution of atmospheric carbon dioxide (CO₂) is important for understanding the carbon natural cycle, predicting its future levels and its impact on global warming and climate changes. Laser technology has advanced considerably during the past few years in the 2-micron region where strong optimum lines are available for measuring CO₂ using the Differential Absorption Lidar (DIAL) technique. Although several types of detectors might be suitable for this particular wavelength, an ideal device would have high gain, low noise and narrow spectral response peaking around the wavelength of interest. This increases the signal-to-noise ratio and minimizes the background signal, thereby increasing the instrument sensitivity and dynamic range. In this paper the detector requirements for a long range CO₂ DIAL measurement will be presented. The requirements were compared to commercially available and newly developed infrared (IR) detectors. The IR detectors considered for this study consist of the well developed InGaAs and HgCdTe p-n junction photodiodes, beside the newly developed and proposed InGaAsSb and InGaSb detectors. All of the detectors were characterized and their performances were compared with the CO₂ DIAL detector requirements. The characterization experiments included spectral response, dark current and noise measurements. CO₂ DIAL measurements using InGaAs detectors were attempted and indicated the need for better detector performance. While InGaAs detectors showed the closest performance to the instrument requirements, InGaSb detectors indicated a promising solution.

Observing system simulation experiments (OSSE's) provide an effective means to evaluate the potential impact of a proposed observing system, as well as to determine tradeoffs in their design, and to evaluate data assimilation methodology. Great care must be taken to ensure realism of the OSSE's, and in the interpretation of OSSE results. All of the OSSE's that have been conducted to date have demonstrated tremendous potential for space-based wind profile data to improve atmospheric analyses, forecasts, and research. This has been true for different data assimilation systems, analysis methodology, and model resolutions. OSSE's clearly show much greater potential for observations of the complete wind profile than for single-level wind data or observations of the boundary layer alone.

Planning is in progress to launch a much improved temperature and moisture sounder called GIFTS- Geosynchronous Imaging Fourier Transform Spectrometer. The IPO of the NPOESS had developed an Airborne Sounder Test bed, NAST, to simulate GIFTS data products. The IPO has also developed an airborne Doppler wind lidar (Twin Otter Doppler Wind Lidar - TODWL) to provide accurate wind profiles over the oceans to enable evaluation of the GIFTS and other space-based wind observing systems. This presentation reports on the first in a series of TODWL under flights of the NAST flown on NASA’s ER-2.

Observing System Simulation Experiments (OSSE) conducted by organizations and reseachers around the world indicate that accurate global wind profiles observed by a spaceborne Doppler wind lidar (DWL) have the potential to significantly improve weather forecasting, hurricane tracking, and global climate studies. Accurate wind profiles from airborne and spaceborne platforms will also have national defense and homeland security applications. In this paper, we will first give a brief review of the history and status of Doppler wind lidar development. Then we will present some results from GroundWinds, a ground-based direct detection Doppler wind lidar (D³WL) technology development and demonstration testbed sponsored by the National Oceanic and Atmospheric Administration (NOAA). We will describe our plan for observing winds from 30 km looking down as part of the BalloonWinds program. We will then use GroundWinds as references to discuss the feasibility and requirements for a spaceborne D³WL in the context of an initial point design. We will discuss Raytheon's internal research and development (IRAD) plan with the objective of developing a prototype space-qualified laser as an engineering model and risk reduction laser for a spaceborne Doppler wind lidar.

As we plan for a future space-based wind lidar, there are several data product issues that are only resolvable with airborne downward scanning lidars. The US Navy has developed an airborne coherent Doppler lidar that allow us to perform a number of experiments to address some of these issues. During field programs in 2002 and 2003, flights over the ocean near Monterey, California revealed the frequent existence of organized circulations that contained correlations between the aerosol backscatter and the variations in the wind field. This paper discusses the instrument and some of the data that were collected in the presence of organized large eddies in the marine boundary layer.

Ethylene has been monitored with a single-ended CO₂-TEA laser-based DIAL system using a topographic target. The direct correlation between ozone concentration and ethylene/NO_x ratio were demonstrated. Our work brings an additional confirmation of concurrent VOC/NOx and NOx-limited regimes in the generation of excess ozone.

We report standoff open path atmospheric CO2 monitoring with a field deployable, turn key system including a continuous wave (CW)distributed feedback (DFB) laser and an erbium doped fiber amplifier (EDFA) at 1.5-μm. A sensitivity of 28-ppm was achieved over 1.5-km of open air with 200-pW of received power, a 10s acquisition time, and a peak absorption cross section of 8x10^-23. This sensitivity corresponds to an error in fractional absorbance of 8x10^-3. Closed cell lab sensitivities are better than 3000ppm*m, an error in fractional absorbance of 5x10^-4. These results have been achieved using space qualified laser components, un-cooled InGaAs detectors, off the shelf electronics in a rugged all fiber architecture.

During nearly a decade of remote sensing programs under the auspices of the U.S. Department of Energy (DOE), LLNL has developed a set of performance modeling codes -- called APRS -- for both Active and Passive Remote Sensing systems. These codes emphasize chemical detection sensitivity in the form of minimum detectable quantities with and without background spectral clutter and in the possible presence of other interfering chemicals. The codes have been benchmarked against data acquired in both active and passive remote sensing programs at LLNL and Los Alamos National Laboratory (LANL). The codes include, as an integral part of the performance modeling, many of the data analysis techniques developed in the DOE's active and passive remote sensing programs (e.g., "band normalization" for an active system, principal component analysis for a passive system).

A continuous-wave (CW) NIR carbon-dioxide monitoring system, incorporating Wavelength Modulation Spectroscopy (WMS), has been developed and was tested aboard the Spirit of Goodyear airship platform. The data shows sensitivities nearly identical to previous ground-based tests but with much higher information rates (100Hz). These tests were conducted over regions with varying ground albedo and included path lengths up to 1.5 km. The system utilized commercial-off-the-shelf (COTS) components including telecom laser diodes and amplifiers. Currently, the system is limited by Erbium Doped Fiber Amplifier (EDFA) spectral bandwdith, but the ever-increasing average power of quantum cascade lasers coupled with the development of midwave fiber technology could make this CW-based architecture a viable solution for future airborne sensors in the MWIR region.

The prototype of the dual He-Ne/He-Xe laser system for gas detection using differential absorption of radiation backscattered from topographic targets is described. Both lasers were excited by dc discharge and for lengths of 80 cm we obtained the output power of about 10 mW. Using receiver optics with the diameter of 7 cm and thermocooled HgCdTe detector we can measure the presence of methane on the distance up to 50 m. The new solution is under construction. to increase the range of measurement, the Casseigrain optics with diameter of 25 cm is being prepared. Using the special construction of RF excited gas lasers with the output power of 40 mW, the measurement distance of 100 m is expected.

Water vapor measurements have been added to the aerosol/temperature lidar operated by the NOAA/Climate Monitoring and Diagnostics Laboratory at Mauna Loa Observatory (MLO). The 532 nm light from an Nd:YAG laser is used and two channels measure the raman shifted light at 607 nm (nitrogen) and 660 nm (water vapor). The receiver is a 74 cm diameter parabolic mirror with the two detectors at the prime focus. An interference filter and two high pass filters achieve a rejection of the 532 nm light of about 1E9, which is needed for measurements of water in the upper troposphere where the water mixing ration can be a few parts per million. Radiosonde flights from the observatory were used for both the calibration constant and the low altitude overlap corrections. The sonde flights used both Vaiasala humidity sensors and chilled mirror hygrometers. The Vaiasala sensors were accurate to about 11 km (-50°C). The chilled mirror hygrometer detection limit is determined by the temperature depression attainable by the cooler. The lidar system has been used for validation of the Atmospheric Infrared Sounder (AIRS) on the NASA/Aqua satellite launched in May, 2002.

Several lidar campaigns have been performed in support of calibration/validation of DMSP SSM/T-2 microwave water vapor sensors. Calibration capabilities were demonstrated by performing radiative transfer calculations based on water vapor profiles measured by lidar. The calculations agreed with collocated SSM/T-2 atmospheric channel measurements to better than 1K RMS, whereas discrepancies were frequently greater than 2 K for radiative transfer based on conventional AIR and Vaisala radiosonde profiles. The improved capability is attributed to the new ability to measure water vapor from 8 to 14 km altitude. Conventional radiosondes tend to be unresponsive to water vapor at the low temperatures typical of altitudes above 8 km.

In this paper, as far as we know, it is the first time that a novel acousto-optic pure rotational Raman lidar based on acousto-optic tunable filter (AOTF) is put forward for the application of atmospheric temperature measurements. AOTF is employed in the novel lidar system as narrow band-pass filter and high-speed single-channel wavelength scanner. This new acousto-optic filtering technique can solve the problems of conventional pure rotational Raman lidar, e.g., low temperature detection sensitivity, untunability of filtering parameters, and signal interference between different detection channels. This paper will focus on the PRRS physical model calculation and simulation optimization of system parameters such as the central wavelengths and the bandwidths of filtering operation, and the required sensitivity. The theoretical calculations and optimization of AOTF spectral filtering parameters are conducted to achieve high temperature dependence and sensitivity, high signal intensities, high temperature of filtered spectral passbands, and adequate blocking of elastic Mie and Rayleigh scattering signals. The simulation results can provide suitable proposal and theroetical evaluation before the integration of a practical Raman lidar system.

A high-quality gadolinium vanadate (GdVO₄) crystal of 7 at.% thulium (Tm) in the starting material was grown by the Czochralski technique. The measured absorption spectra exhibited sufficient absorption coefficients for LD pumping: 6.0 cm^-1 for π polarization and 6.2 cm^-1 for σ polarization. Laser oscillation was carried out using single-stripe, 808-nm LDs in an end-pumping configuration. A slope efficiency of 28% and a threshold of 750 mW were exhibited with respect to the absorbed pump power. An output power of 420 mW was achieved at the absorbed power of 2.4 W. It was demonstrated that Tm:GdVO₄ is actually useful material for 2-μm lasers, particularly in the compact LD-pumped system.

In general Asian dust storms occurring during the spring season in the northeast Asia play an important role in radiative forcing and regional climate change. In order to investigate the characteristic of optical properties of Asian dust particles atmospheric aerosol vertical profile was measured with a multi-wavelength LIDAR system developed by ADEMRC, K-JIST and a collocated micro-pulse LIDAR (MPL) during the ACE-Asia intensive observation period, 11 March ~ 4 May 2001 at the Gosan super site (33°17'N, 126°10'E) in Jeju Island, Korea. Air mass backward trajectory analysis shows that air masses came from either the northwestern Chinese desert regions or northeastern Chinese sandy areas. It has been shown that combining the LIDAR data and back trajectory analysis can assess the transport characteristics of atmospheric aerosol during the Asian dust events. The LIDAR-derived aerosol optical depth values were compared with those measured by a collocated AERONET sun photometer. Relationship between the LIDAR data and chemical data of atmospheric particulate matters observed at the surface has been analyzed.

A Raman lidar dedicated to night-time tropospheric water-vapor high-resolution measurements is currently being developed at Réunion island in the south-western Indian Ocean. To our knowledge, it is the first permanent instrument of its kind in this tropical region. The geophysical and instrumental interests and issues on the radiative, dynamical and chemical plans for such a measurement, specially in the tropics, are obvious. The choice of a visible laser excitation wavelength was initially a constraint, in view of the weakness of the Raman scattering process that is the basis of the development of this instrument, but many arguments also plead for such a choice. After describing the water-vapor measurement method of this lidar, which is straightforward in principle, we stress on the main delicate underlying issues related to this method. A precise description of the optical parts of the lidar system is then given that emphasizes the importance of the rejection of the elastically backscattered signals in the Raman channels. Finally, we list the most important future works concerning the validation and calibration stages of this instrument that is intended to become an atmospheric surveillance instrument on a medium term.

The use of multi-wavelength lidar measurements can in principle be used to estimate aerosol extinction and backscatter profiles. From the resultant multi-wavelength profiles, an estimate of particle size distribution parameters can be inferred. However, the confidence of the retrieval depends on the accuracy of the retrieved backscatter and extinction coefficients. Conventional approaches determine errors by performing statistical calculations on the results of inversions of simulated measurements with errors derived from a random number generator. There is a certain ambiguity about the validity of this method which we try to resolve. We introduce two alternative methods. One is a reverse Monte-Carlo method which gives an ensemble of distribution parameters that produce simulated measurements whose inversions match the inversion of the retrieved measurements. Substantial differences in the results of the aforementioned methods were found. In order to clarify these differences, a semi-analytic approach is introduced that formulates the conditional probability density function of underlying particle size distribution parameters given a set of measurements (from the combined probability function with the underlying optical data). Results using the probability density function were found to be closer to the statistics of the reverse Monte-Carlo method than the conventional method.

Multiwavelength elastic lidar is often used to probe the aerosol profiles of the atmosphere. Normally, the atmosphere is considered homogeneous and an a-priori aerosol ratio is given for each wavelength channel which is then processed independently. However, it is clear that the multiwavelength retrieved backscatter profiles should contain information that can be used to estimate particle size distribution which may provide a new estimate to range dependent aerosol ratio profiles which can be repeated until convergence. In this paper, we illustrate the basic idea of using multiwavelength data using a two wavelength lidar to obtain local information on the lidar ratio which can be used to improve lidar profiling in homogeneous atmospheres and show that a key feature of any scheme is the monotonic dependence between the optical data ratio and the distribution parameter. In addition, we extend the approach to a prototypical Nd:YAG three wavelength (355, 532, 1064 nm) lidar arrangement and show that while an iterative lidar procedure can be used to extract range dependent profiles, imprecision in the inversion procedure as well as error propagation of the lidar back integration can hamper convergence.

The GroundWinds photon recycling fringe imaging direct detection Doppler LIDAR systems have been used to validate models of systems using this technology. These instruments have been characterized extensively over the past 2 years in an effort to experimentally determine the performance enhancements that the GroundWinds technology provides. This effort focused on the validation of all aspects of the instrument performance, including component and system transmissions, photon recycling gains, camera and system noise sources, and background contribution. The results of these investigations have been used to formulate a point design for a space-based system. Presented here are the performance predictions and point design parameters for a spaceborne Doppler wind lidar that utilizes the GroundWinds fringe imaging technology.